While the present invention has broad application in many fields, such as communications and power transmission, pulsed power and electro-optics, it is especially suitable for use in the generation of an extremely high power microwave pulse(s) (a burst of microwave energy), preferably in the form of several cycles of a periodic sine or square wave, in the GHz regime. While the examples discussed below refer to microwave generation, this does not imply any restriction as to the applications to which the present invention may be put.
The general concept of producing microwaves by a sequential operation of switches is well known. High peak power microwave generation is addressed by Driver et al. in U.S. Pat. No. 4,176,295 in which the generation of microwaves by periodically discharging a plurality of identical, direct current energized, resonant transmission lines into a TE wave guide at half-multiple wavelength spacings is discussed. To periodically discharge the transmission lines, each is provided with a light activated solid state (LASS) diode switch, the LASS diode switches being simultaneously operated by laser beams of equal optical path length to cause the electromagnetic energy in the waveguide to propagate as a pulse train of microwave energy
Mourou, in U.S. Pat. No. 4,329,686 discusses an arrangement, similar to that of Driver et al., which uses a TE waveguide and a LASS switch for generating microwave pulses of picosecond duration, synchronously and in response to laser light pulses.
Unfortunately, the arrangements described by Driver et al. and Mourou do not produce clean microwave pulses and are limited in power since TE waveguides have impedances close to that of free space, typically 50 ohms or more, and therefore cause the LASS switches to operate outside the electric field and current density limits consistent with good high power design principles, specifically, unidirectional power flow in a continuously matched system.
In "Experimental demonstration of high power, fast rise-time switching in silicon junction semiconductors" Applied Physics Letters, Volume 29, page 261, Zucker, Long, Smith, Page and Hower discuss the use of a lightactivated semiconductor switch, the basic principle of which is to create carriers in situ, thus obviating the need for diffusing the carriers necessary to transition a transistor or thyristor switch from a reversed biased (OFF) condition to a forward biased (ON) condition. In a later publication, "Light Activated Semiconductor Switches," UCRL Preprint, Oct. 1977 Zucker and Long discuss the use of a laser beam whose frequency is matched to the switching device bandgap (1.12eV for silicon) to turn ON a LASS switch in less than 1 ps. As discussed in the article, a switch having sub-nanosecond turn on time, and capable of fast turn off after current ceases to flow, would be required for microwave generation in order to allow for quick recharge and refire and for the establishment of coherence among independent microwave sources.
Such a switch is addressed by Proud et al. in their article "High Frequency Waveform Generation Using Optoelectronic Switching in Silicon" IEEE Trans on Microwave Theory and Techniques, Vol. MTT-26, No. 3 (1978), in which the conversion of dc energy into RF pulses by using an array of silicon switches simultaneously activated by a laser pulse is discussed. Proud et al. envision a frozen wave generator comprising array of high-resistivity silicon switches fired by a gas laser designed to simultaneous fire all of the switches in exact synchronism.
Mourou et al. in their article entitled "Picosecond Microwave Pulse Generation", Applied Physics Letters 38(6) (1981) discuss the generation of a microwave burst in picosecond synchronization with an optical pulse using a LASS switch coupled to an x-band waveguide and describe the efforts of others to generate microwave pulses using electrically driven spark gaps and frozen wave pulses using LASS switching.
The LASS switch can take on several forms, depending on such factors as the level of doping within the semiconductor, the profile of the doping, the amount of optical energy supplied to the device in order to turn on and the direction at which the optical energy is introduced to the switch with respect to the electric field within the switch.
D. H. Auston, in "Picosecond optoelectronic switching and gating in silicon", Applied Physics Letters, Volume 26, page 101 (1975), illuminated a gap in a microstrip transmission line, which was laid down on a silicon substrate, by a laser pulse with wavelength .lambda.=0.53 .mu.m. This created a thin layer of electrical carriers in the upper portion of the substrate within the gap, thus allowing current to flow along the microstrip line. Current through the line was later stopped by illuminating the gap with a laser pulse of wavelength .lambda.=1.06 .mu.m which created a region of carriers which extended throughout the height of the silicon substrate, effectively shorting the upper conductor to the lower (ground) conductor. The voltage switched was 35 V into 50 .OMEGA. in a time of 15 ps, thus acting as a fast switch for low power pulses. This structure is the subject of U.S. Pat. No. 3,917,943.
LeFur and Auston, in their article "A Kilovolt Picosecond Optoelectronic Switch and Pockel's Cell" Applied Physics Letters, Volume 28, No. 1 (1976) pages 21-23, which discuss a silicon switch which is turned on by absorption of a 5 psec optical pulse from a mode locked Nd:glass laser. LeFur and Auston contemplate the combination of a silicon switch and Pockel's cell in order to efficiently switch large optical signals by small optical signals at high speed. The voltage applied across the gap was increased to 1.5 kV and was applied in a pulsed mode, rather than d.c. as in the earlier switch. An estimated 45 kW of electrical power was switched using this technique (1.5 KV into 50 .OMEGA.).
G. Mourou, in U.S. Pat. No. 4,347,437, describes a semiconductor switch which employs avalanche breakdown. In this design of a LASS switch, the high voltage held across the switch is less than the voltage required to cause avalanche breakdown. Avalanche occurs when the energy of a carrier accelerated by the electric field is such that impact ionization occurs on collision with an atom in the semiconductor lattice. A small concentration of carriers is generated uniformly throughout the semiconductor by an optical pulse, thus triggering the avalanche process. The device has to be kept at cryogenic temperatures to reduce the possibility of a thermally generated carrier initiating the avalanche process. Mourou claims that a laser diode pulse of 3 nJ can produce 1 MW of power using this method.
S. J. Davis, in U.S. Pat. No. 4,438,331, describes a semiconductor switch, fabricated from intrinsic semiconducting material, with some light, uniform doping to trap thermally generated carriers, which holds off up to 10 kV over a microstrip gap of 1-3 mm. The size of the switch is not given, except for the fact that the switch itself is rectangular in shape, with a length which is at least twice that of the microstrip gap. The switch is activated by a diode laser with an optical pulse length of the order of 500 ps. It was claimed that such a device could switch 2 MW (10 kV into 50.OMEGA.) in sub-nanosecond time scales, with an optical pulse in the energy range of 10-100 nJ.
In a subsequent patent, U.S. Pat. No. 4,864,119, L. O. Ragle and S. J. Davis describe the use of partial light penetration in a LASS to cause field enhancement and subsequent avalanche. The preferred embodiment of this patent included a LASS in which the activating optical pulse was introduced into the semiconducting material in a direction parallel to the electric field (e.g., through a hole in an electrode). The electric field held across the device is between one tenth and one third that field needed to induce avalanche. The wavelength of the light and the band-gap of the semiconductor were chosen such that the optical pulse was substantially absorbed before reaching the second electrode. This creates a volume of material which has low resistance due to photoconduction and so the voltage of the first electrode is transferred to the surface of the conducting region. The field held across that part of the switch which is still insulating is therefore enhanced to a point where avalanche breakdown occurs and a current "breaks through" to the second electrode, thus closing the switch. Since a significant fraction of the switch volume requires to be photoconducting, the optical requirements of this type of switch are significantly higher than for the Mourou-type switch. It is claimed that a voltage of 5 kV can be switched by an optical input in the range of 20-200 nJ.
L. O. Ragle, S. J. Davis and R. A. Williams, in U.S. Pat. No. 4,864,119, subsequently described a switch of the field enhancement/avalanche design using a mesa structure for the top electrode in order to improve the voltage hold off capabilities.
The virtues of LASS switches over other high power switches such as the spark gap and SCR has long been recognized. The spark gap has a high power handling and a fast current rise time capability relative to the SCR but is slower than a LASS switch and is short lived. The conventional semiconductor switch has the ability to handle moderately high powers and is long lived but is relatively slow since it relies on charge carriers diffusing laterally into a junction for switching. By means of optical carrier generation, LASS switches in essence provide a switching action such as that found in thyristors or other junction devices with a current rise time capability in the nanosecond to picosecond range and thus combine the junction device high power handling capability and long life with fast rise time.
There are many different designs for photoconductive switches, as illustrated by the discussion above. However, these designs make little or no attempt to increase the power handling capability. There is an unfulfilled need for a LASS design which optimizes the power which can be transferred by the switch. The basic difference between the present invention and prior art is that the present invention takes into account the dimensional and impedance relationships which allows for maximum power generation. This is the impedance relation.
Moreover, while various schemes for generating microwaves using LASS switches (a.k.a. photoconductive solid state or "PCSS" switches) are known, no truly effective digitally synthesized microwave sources are presently available. In addition, there exists a need for a microwave source which can project significant amounts of microwave energy at a predetermined point in space, a need which requires a plurality of individual sources timed to be coherent with one another, a need not satisfied by prior art devices. There also exists an unfulfilled need for a microwave source which can produce either continuous microwave energy or short bursts of microwave energy of high magnitude. Further, no available high power microwave sources have sufficient intersource coherence to generate phase coherent microwave pulses from a phased array of microwave sources.